Optimal cut for saw devices on langasite

ABSTRACT

A lanthanum gallium silicate single crystal substrate, referred to as langasite, has a prescribed range of Euler angles for substrate and crystal orientation for improving signal processing for a surface acoustic wave (SAW) device. When a voltage is applied to an input interdigital transducer (IDT) of the SAW device, a surface acoustic wave is generated in the langasite piezoelectric substrate. The surface acoustic wave propagates in a direction generally perpendicular to electrodes of the IDT. The langasite crystal cut and wave propagation directions are defined which reduce insertion loss due to SAW transduction, diffraction, and beam steering. As a result, temperature stability for the SAW device is improved. A low power flow angle and reduced level of diffraction is also achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending InternationalApplication No. PCT/US96/17906, filed Dec. 20, 1996, claiming priorityto Russian application No. 96-100012/09, filed Jan. 10, 1996, commonlyowned with the present invention.

FIELD OF THE INVENTION

The invention relates to a surface acoustic wave (SAW) device and, moreparticularly, to a device having a langasite crystal substrate with apredetermined crystalline orientation for causing a surface acousticwave to propagate along a predetermined crystalline axis of thesubstrate.

Background of the Invention

This invention relates to an optimal surface acoustic wave orientationon single crystal lanthanum gallium silicate or La₃ Ga₅ Si O₁₄, commonlyknown as langasite (LSG). SAW devices are currently used as bandpassfilters, resonators, delay lines, convolvers, etc. in a broad range ofRF and IF applications such as wireless, cellular communication andcable TV. Three commonly used substrates are lithium niobate, ST-Quartz,and lithium tantalate. There are several material properties thatdetermine the usefulness of any particular crystal and, in particular,orientation of a particular crystal. These properties include: 1) SAWvelocity, Vs; 2) the SAW piezoelectric coupling coefficient, k² ; 3) thepower flow angle, PFA; 4) the diffraction or beam spreading coefficient,γ (gamma); and 5) the temperature coefficient of delay, TCD. It has notbeen possible to find an orientation in any existing substrate whichoptimizes these properties at the same time; so the choice of substrateand orientation depends upon what is important for the application, andalmost always involves a compromise between the SAW material properties.A high velocity is desirable for high frequency devices, because thedevice geometry patterns are larger and, therefore, the devices areeasier to fabricate. At low frequencies, a low velocity is usuallydesirable because the device size is smaller, resulting in lower devicesand packaging costs. Thus, there is no universally optimum velocity. Formoderate to wide bandwidth devices, a high value of k² is desirablebecause it allows lower insertion loss. From the point of view of k²,lithium niobate is best, quartz is worst, and lithium tantalate is inbetween. For most devices, and in particular narrow band devices, TCDshould be as low as possible and ideally zero. From this point of view,ST-Quartz is best, lithium niobate is worst, and lithium tantalate is inbetween (just the opposite ranking as for k²). The optimum value of γ is-1, which results in minimum beam spreading. From this point of view, YZlithium niobate is now ideal, ST-Quartz is worst, and lithium tantalateis in between. The PFA should be zero, and this is the case for most ofthe commonly used SAW substrates, with an exception being 112° lithiumtantalate, which has a PFA of 1.55°. For the most narrow bandapplications, ST-Quartz is a quite acceptable choice; and for the verywide band applications where temperature stability is not so important(e.g., a device can be held at constant temperatures), lithium niobateis usually quite acceptable. What is needed is a substrate orientationthat offers the temperature stability of ST-Quartz but with higher k²and at the same time low or zero beam steering (PFA) and diffraction(γ=-1). It is an object of the present invention to provide a substratethat meets these conditions.

Earlier research looked at using langasite to achieve a favorabletrade-off in the material properties that determine the coupling, PFA,diffraction, and TCD. The choice of Euler angles were as follows: φ=90°,θ=10°, and ψ=0°. The performance parameters of this orientation werequite poor. The TCD is 12 ppm/° C. (ST-Quartz is near zero), thecoupling k² =0.26% (better than ST-Quartz, which is 0.11%), PFA=-5.7°(ST-Quartz is zero), and diffraction coefficient γ=-2.86 (also worsethan ST-Quartz). Only the coupling is better than ST-Quartz.

A different orientation of langasite has been formed which has farsuperior properties to both ST-Quartz and the earlier orientation oflangasite.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a langasite crystalhaving a substantially planar surface defined by the Euler angles, suchthat SAW propagation within this range of angles on langasite ischaracterized by piezoelectric coupling about two to three timesstronger than typically found with ST-Quartz, by way of example, nearzero PFA, near zero TCD, and near minimum beam spreading. While thelatter three conditions are not all met exactly for any orientation, theperformance as measured just by these three conditions together issignificantly better than any known cut of Li Nb O₃, Li Ta O₃, orquartz, and choices within this range can be chosen to favor one or twoat the slight expense of the third.

One preferred embodiment of the present invention includes a devicewhich contains a langasite substrate on the surface of which input andoutput IDT's are placed.

This purpose is achieved as follows: the surface of the langasitesubstrate is perpendicular to axis Z', electrodes of IDT's areperpendicular to axis X' and are parallel to axis Y'. Axes X', Y', Z'are defined by Euler angles with respect to crystal axes X, Y, Z oflangasite. Angle φ is in the range -15° to 10°; angle θ in the range of120° to 165°; and angle ψ in the range of 20° to 45°.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a SAW device illustrating interdigitaltransducers located on a langasite substrate surface, and a power flowangle Φ;

FIG. 2 diagrammatically illustrates substrate axes X', Y', and Z' andcrystal axes X, Y, and Z along with Euler angles φ, θ, ψ describingrelative orientation of X,Y, and Z to X', Y', and Z', wherein X' isdefined as a direction of SAW propagation;

FIGS. 3A-3D illustrates SAW parameters (Velocity, Power Flow Angle,Electromechanical Coupling Coefficient, and Temperature Coefficient)versus propagation angle ψ for different values of θ; and

FIG. 4A-4D illustrates SAW parameters of FIG. 3A-3D versus ψ for θ=145°for different values of φ.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated, by way of example with reference to FIG. 1, onepreferred embodiment of the present invention includes a SAW device 10which contains a langasite substrate 12 on the surface 14 of which aninput interdigital transducer and an output interdigital transducer(IDT) are placed.

The surface 14 of the langasite substrate 12 is perpendicular to axisZ', electrodes 20, 22 of IDT's 16, 18 respectively, are perpendicular toaxis X' and are parallel to axis Y'. As illustrated with reference toFIG. 2, axes X', Y' and Z' are defined by Euler angles with respect tocrystal axes X, Y and Z of the langasite substrate 12. For the preferredembodiment of the present invention, angle φ is in the range -15° to10°; angle θ in the range of 120° to 165°; and angle ψ in the range of20° to 45°.

The crystal cut of langasite with Euler angles φ=-1.8°±10°,θ=135.6°±10°, and ψ=24.1°±10°, provide improved performance for SAWdevices. Specifically, the crystal cut provides a near simultaneousoptimization of three critical SAW propagation parameters and afavorably value of a fourth parameter. This fourth parameter is thecoupling constant k², which varies between 0.25% and 0.35% as comparedto 0.12% for ST-Quartz crystal. The three SAW propagation parameters arethe PFA, γ and TCD, which, as earlier described, are the power flowangle, the diffraction coefficient, and the temperature coefficient ofdelay, respectively. PFA is also known as Φ, the beam steering angle,and is the angle between the SAW wave vector, which is normal to the tapelectrodes, and the direction of the power flow, as illustrated againwith reference to FIG. 1. Ideally, the PFA would be zero. γ is a measureof the diffraction or beam spreading. Normally, as a SAW propagates on asubstrate, the beam profile will change and broaden. This beam spreadcauses diffraction loss and distortion to the filter response. Forisotropic materials, the value of γ is zero, and diffraction is amoderately serious problem. Diffraction is minimized when γ=-1, and thisis the case for YZ Li Nb O₃ and a special MDC (minimum diffraction cut)of Li Ta O₃. For ST-Quartz, γ=+0.38, and diffraction is worse than theisotropic case. There is a range of angles within the designated rangeof this disclosure for which γ=-1, which is ideal. Likewise there is arange of angles for which the TCD is zero. (TCD is the relative changein delay per degree centigrade.) The desired parameter values areobtained for each parameter within the restricted range of angles ofthis disclosure; but since the angles associated with the values form alocus of points in a two-dimensional angle space (over θ and ψ), it isvery difficult to find a point at which the three loci intersect. Thatmeans it is possible to achieve a desired performance in two of thethree parameters and nearly ideal performances for all three parameters.Therefore, within this range, the optimal choice of angles would stillbe dependent upon the application, and in fact are intermediate pointsthat minimize the problem of all three parameters. This is the reasonfor the spread of angles disclosed herein.

The Euler angle convention used is as described by Slobodnik et al. in"Microwave Acoustic Handbook," Vol. 1, Surface Wave Velocities,AFCRL-70-0164, March 1970, Physical Sciences Research Papers, No. 414,Office of Aerospace Research, USAF.

Consider a semiconductor wafer outline on a surface normal to the axisZ'. Now construct a flat plane along one edge of the wafer which isnormal to the axis X'. The direction of SAW propagation is parallel toaxis X'. Now assume that the crystal axes X, Y, Z are coincident withthe wafer outline axes X', Y', Z', respectively. With no rotation, thewafer is considered a Z cut (the wafer is cut with the polished surfacenormal to Z) and X propagating (the SAW propagates in a directionparallel to the X axis). With any subsequent rotation, the wafer axesX', Y', Z' are rotated, and the crystal axes X, Y, Z are assumed to befixed. Now, by way of example, consider the Euler angles (φ, θ, ψ)=(0°,135°, 28°), which is a case near the middle of the designated range. Thefirst rotation would rotate around Z' (X' toward Y') by φ. Since φ=0°,there is no rotation for this case. The next rotation is around the"new" X' (the "new" axes are always tied to the wafer so that anyrotation is around a wafer axis that includes all previous rotations) byθ (which is 135°) (Y' toward Z' for a positive angle rotation). Finally,rotate around Z' (X' toward Y') by ψ, which for the case hereindescribed is 28°.

By way of further example with regard to an orientation procedure fordefining a substrate (also referred to as a wafer) using Euler angles,begin with axes X, Y, Z as the crystal axes (also referred to as a bouleaxes) coincident with the substrate axes X', Y', Z'. The relationshipbetween X, Y, Z and X', Y', Z' is independent of the overall orientationof its combined system in space. In preparation for cutting the crystal,and as viewed from the positive Z' axis (now coincident with thepositive Z axis), first rotate the substrate φ° counterclockwise aroundits axis Z'. Second, as viewed from the positive X' axis, rotate thesubstrate counterclockwise by θ° about the substrate axis X'. Next, asviewed from the positive Z' substrate axis, rotate the substratecounterclockwise by ψ° about the substrate axis Z'. The crystal is nowprepared for a cut normal to the substrate axis Z', and a flat definingthe direction of propagation is placed normal to the wafer axis X' alonga substrate edge in the positive X' direction.

FIGS. 3A-3D illustrates SAW velocity, PFA Φ, electromechanical couplingk² and temperature coefficient versus Euler angle ψ for some values ofangle θ and for φ=0°. Velocity, Φ, k² and temperature coefficient versusψ for various values of θ. By way of example, these same parameters areillustrated versus ψ for θ=145° in FIGS. 4A-4D for various values of φ.

Again with reference to FIG. 1, and by way of example, one preferredembodiment of the present invention includes the SAW device 10containing the langasite substrate 12, and IDT's 16, 18 and reflectingelectrodes 24, 26. As earlier described, the axis Z' is normal to thesubstrate surface 14, the axis X' is normal to electrodes 20, 22, andthe Y' axis is parallel to the electrodes 20, 22. These axes X', Y' andZ' are defined with respect to crystal axes as follows: φ=-15° to 10°;θ=120° to 165°, ψ=20° to 45°, where φ, θ, ψ are the Euler angles.

With reference again to FIG. 2, φ is the angle between crystal axis Xand auxiliary axis X", which is the axis of rotation of the plane XY (upto required orientation of the substrate surface).

θ is the angle between axis Z and the normal Z' to the substrate surface14.

ψ is the angle between axis X" and axis X', X' is perpendicular toelectrodes of IDT's 20, 22.

Advantages of the device 10 as compared to earlier proposed cuts forlangasite are realized. By way of example, SAW propagation parameters onlangasite with orientation (90°, 10°, 0°) would be as follows:temperature coefficient=12 ppm/° C., k² =0.26%, Φ=-5.7°.

As illustrated again with reference to FIGS. 3 and 4 for the presentinvention, based on a langasite cut for the Euler angles within chosenlimits 15°≦φ≦+10°, 120°≦θ≦165°, 20°≦ψ≦45°, the temperature coefficientdoes not exceed 10 ppm/° C.; and for orientation (0°, 135°, 24°), it isclose to zero.

As a consequence, the temperature stability is improved as compared tothe prior works completed on langasite.

Again with reference to FIGS. 3A-3D and 4A-4D, it is shown that in thepresent invention, the PFA, Φ is less than 5° and the electromechanicalcoupling coefficient is more than 0.2% with the maximum value 0.45%.Consequently for the orientations of the present invention, theelectromechanical coupling coefficient k² is more than twice, and forsome cases up to four times, that of earlier devices on ST-Quartz. Thereare several selections of orientations for which the PFA issubstantially zero and the diffraction parameter γ is near the optimalvalue of -1. Additionally, the TCD in this range of orientations is ator near zero.

While specific embodiments of the invention have been described indetail herein above, it is to be understood that various modificationsmay be made from the specific details described herein without departingfrom the spirit and scope of the invention as set forth in the appendedclaims.

Having now described the invention, the construction, the operation anduse of preferred embodiments thereof, and the advantageous new anduseful results obtained thereby, the new and useful constructions,methods of use and reasonable mechanical equivalents thereof obvious tothose skilled in the art, are set forth in the appended claims.

What is claimed is:
 1. A surface acoustic wave (SAW) device comprising:alangasite substrate having a SAW propagation surface; input and outputinterdigital transducers having electrodes on the surface for launchingand detecting surface acoustic waves, wherein a surface wave directionof propagation is along an X' axis, the substrate further having an Z'axis normal to the surface, and a Y' axis along the surface andperpendicular to the X' axis, the langasite substrate further having acrystal orientation defined by crystal axes X, Y, and Z, the relativeorientation of device axes X', Y', and Z' being defined by Euler anglesφ, θ, and ψ; and wherein φ has a value ranging from -15° to 10°, θ has avalue ranging from 120° to 165°, and ψ has a value ranging from 20° to45°.
 2. The device according to claim 1, wherein the Euler angle φ iswithin the range of -1.8°±5°, θ is within the range of 135.6°±5°, and ψis within the range of 24.1°±5°.
 3. The device according to claim 1,wherein the Euler angle φ is proximate 0°, θ is within the range of135°±10°, and ψ is within the range of 30°±10°.
 4. A surface acousticwave device comprising:a substrate having a substantially planar surfacefor propagating surface acoustic waves thereon, the substrate formedfrom a single lanthanum gallium silicate (langasite) crystal cut forproviding a crystal orientation for forming the surface defined by Eulerangles having a range of -10°≦φ≦10°, 120°≦θ≦165°, and 20°≦ψ≦45°; andinterdigitized electrodes affixed to the surface.
 5. The deviceaccording to claim 4, wherein the electrodes have an orientationgenerally perpendicular to a direction of wave propagation generallydefined by the crystal orientation Euler angle ψ.
 6. A surface acousticwave device substrate formed from a langasite single crystal, thesubstrate comprising:a planar surface for an interdigital transducerhaving a plurality of electrodes, the interdigital transducers forlaunching and detecting surface acoustic waves propagating generallyperpendicular to the electrodes; and a crystal orientation cut forforming the surface, the crystal orientation defined by Euler angles φ,θ, and ψ, wherein φ has a value ranging from approximately -10° toapproximately 10°, θ has a value ranging from approximately 120° toapproximately 165°, and ψ has a value ranging from approximately 20° toapproximately 45°.
 7. A method for forming a surface acoustic wavedevice comprising the steps of:providing a lanthanum gallium silicate(langasite) single crystal; orientating the crystal for cutting a planarsurface, the crystal orientating defined by Euler angles φ, 74 , and ψ;cutting the crystal for forming the planar surface, the cut defined bythe Euler angles, wherein φ has a value ranging from approximately -15°to approximately 10°, θ has a value ranging from approximately 120° toapproximately 165°, and ψ has a value ranging from approximately 20° toapproximately 45°; and affixing an interdigitized transducer on thesurface for propagating and detecting surface acoustic waves propagatingin a direction generally along an axis of propagation defined relativeto the crystal orientation angle ψ.
 8. The method according to claim 7,wherein the transducer affixing step further comprises the step oforientating electrodes of the transducer generally perpendicular to theaxis of propagation.
 9. The method according to claim 7, wherein theEuler angles comprise Φ having a value proximate 0°, θ having a valueranging from approximately 125° to approximately 145°, and ψ having avalue ranging from approximately 14° to approximately 34°.
 10. A methodfor forming a surface acoustic wave device substrate comprising thesteps of:providing a lanthanum gallium silicate (langasite) singlecrystal; orientating the crystal for cutting a planar surface, thecrystal orientating defined by Euler angles φ, θ, and ψ; and cutting thecrystal for forming the planar surface, the cut defined by the Eulerangles, wherein φ has a value ranging from approximately -15° toapproximately 10°, θ has a value ranging from approximately 120° toapproximately 165°, and ψ has a value ranging from approximately 20° toapproximately 45° for affixing interdigitized transducer electrodes onthe surface for propagating and detecting surface acoustic waves in adirection generally along an axis of propagation defined relative to thecrystal orientation angle ψ.
 11. The method according to claim 10,further comprising the step of affixing interdigitized transducerelectrodes on the surface for propagating and detecting surface acousticwaves in a direction generally along an axis of propagation definedrelative to the crystal orientation angle ψ, and orienting theelectrodes generally perpendicular to the axis of propagation.
 12. Themethod according to claim 10, wherein the Euler angles comprise φ havinga value proximate 0°, θ having a value ranging from approximately 125°to approximately 145°, and ψ having a value ranging from approximately14° to approximately 34°.
 13. A method for generally improvingtemperature stability, lowering power flow angle, and reducingdiffraction in a surface acoustic wave device, the method comprising thesteps of:providing a lanthanum gallium silicate (langasite) singlecrystal; orientating the crystal for cutting a planar surface, thecrystal orientating defined by Euler angles φ, θ, and ψ; cutting thecrystal for forming the planar surface, the cut defined by the Eulerangles, wherein φ has a value ranging from approximately -15° toapproximately 10°, θ has a value ranging from approximately 120° toapproximately 165°, and ψ has a value ranging from approximately 20° toapproximately 45°; and affixing interdigitized transducers on thesurface for propagating and detecting surface acoustic waves in adirection generally along an axis of propagation defined relative to thecrystal orientation angle ψ, and wherein electrodes of the transducersare generally perpendicular to the axis of propagation.